This disclosure relates to electric motors and more particularly, to cooled electric motors for operation in environments having elevated temperatures.
Electric motors often must be placed in service in harsh environments. For example, electric motors, typically three-phase alternating current (AC) induction motors, may be used to drive exhaust or cooling fans for the diesel engine enclosures of diesel-electric locomotives. Electric cooling fans may be mounted on the roof of the diesel locomotive to draw ambient air through the radiator, where it absorbs heat from engine and power generation components and exhausts it upwardly. Such fans are thus mounted on the “hot side” of the cooling air that flows through the diesel engine enclosure, so that the exhaust fans typically operate in a stream of heated air from the radiator.
Operating such fan motors in a stream of heated air results in heat build-up within the fan motor enclosures themselves, causing temperatures within fan motor enclosures to reach as high as 190° C. Such elevated motor temperatures may cause the bearing lubricants to degrade rapidly, resulting in increased shear forces, changes in viscosity and elevated lubricant bleed and evaporative rates. Rapid lubricant degradation may cause failure of the rotor bearings after approximately 18-24 months of motor service time, significantly less than the desired motor service time of 72 months.
A desired operating range of motors in such applications is approximately 150° C.-160° C. before failure of the bearing lubricating grease. For every 10° C.-15° C. increase in bearing grease temperature, there is typically a reduction of one half-life of the bearings due to lubricant failure.